This invention relates to torque-transmitting devices, such as torque limiting clutches and torque sensors, in which a torque load is transmitted through rolling elements that are arranged in a manner that is sensitive to overload.
In my earlier application Ser. No. 805 782 filed June 13, 1977 I describe such torque-transmitting devices that comprise a series of pairs of rolling elements in the form of rollers or balls to transmit a torque load between rotary input and output members, the elements of each pair being arranged in tandem in recesses in the members with the load being transferred by a contacting face of the input member recess to a rear element of the pair, then to the front element of the pair to a contacting face of the driven member recess, said contacting faces being so arranged that one of the balls is engaged at a point oblique to the line through the rolling axes of the pair of elements in order to produce a force component transverse to the torque load that tends to cause the elements to counter-rotate on each other and on their engagement with the members. In the case of a torque-limiting clutch, the increase of the torque load above a chosen limit will rotate the elements sufficiently for them to retract into one of the recesses to a release position in which they are disengaged from driving engagement between the members. In the case of a torque sensor, the elements may not themselves move to a release position but they are arranged to actuate warning means and/or stop means, for example when they have been displaced by the limiting load.
It is a feature of the known arrangement that the rolling movement of the elements on disengagement avoids sliding friction at those contact regions of the elements to which the driving torque is being transmitted, so that a more reliable operation can be obtained at a predetermined overload torque value.
The loads that can be transmitted through the pairs of elements are, however, limited by the small contact zones to which the loads are applied. This disadvantage is of course particularly marked if the rolling elements are balls and it has been proposed to use rollers as rolling element pairs instead, but the geometry of the arrangement disclosed requires barrel-shape rollers to avoid introducing sliding friction effects that would reduce considerably the advantages to be obtained from the counter-rotative effect referred to so that the gain in load-carrying capacity is not as great as might be hoped.
There is also another limitation on the load-carrying capacity in this known arrangement, whether balls or rollers are used as rolling element pairs to transmit the torque. Because the contact zones of the elements with the driving and driven members are disposed very close to the edges of the recesses of the members in which the rolling elements are located, the local stress concentrations can only be supported to a limited extent and there can be unwanted plastic deformation of the contact zones on the members at relatively small torque loads.
If the contact zones are arranged further from the edges of the recesses, another problem arises. In order to keep the rolling elements in their operative position against the transverse force component, a backing member must be provided and in the disengaging rolling motion, the elements have to slide on this member. The sliding friction force is reduced due to the fact that the bearing force between the rolling elements and the backing member will be less than the torque load, but it increases with the angle of obliquity of the offset engagement point that produces the transverse force component, and the angle of obliquity will of course be increased by any attempt to increase the spacing of the contact zones axially of the device.